Previously, we observed that hypoxic pulmonary vasoconstriction (HPV) can be reduced by endurance exercise training. This study determined whether nitric oxide (NO) plays a role in inhibition of HPV after endurance exercise training in isolated rat lung perfused with physiological groups : a control group and an endurance exercise-trained group. Endurance exercise training was carried every day on a small-animal motorized treadmill. The training protocol was 30 to 40 min/day and the rats ran at a speed of 15 30 m/min for 2 weeks. It appeared that HPV could be reduced by short-term endurance exercise training. The NO-forming enzyme inhibitor, NG-nitro-L-arginine methyl ester, administered to the trained group increased the HPV compared in that of the control group. These findings indicate that endothelial NO synthesis may contribute to the inhibition of HPV in trained group. Our date suggest that endurance exercise training promotes endothelium dependent-pulmonary vasodilation through the stimulation of NO released during HPV.